The present invention relates to an improved magnet wire, and more particularly, to an improved magnet wire which is highly resistant to repetitive or pulsed, high voltage spikes or surges.
Much has been written over the years about various types of variable frequency or pulse-width modulated (PWM) and/or inverter adjustable speed drives on AC motors and their affect on motor operation. PWM drives are known to have significant harmonics and transients which may alter the motor performance characteristics and life expectancy. The effects of maximum voltage, rate of rise, switching frequencies, resonances and harmonics have all been identified.
The PWM inverter is one of the newest and fastest evolving technologies in non-linear devices used in motor drive systems. The motivation for using PWM inverters is speed control of an AC motor comparable to the prior mechanical or DC adjustable speed drives without loss of torque. With the increased emphasis of energy conservation and lower cost, the use of higher performance PWM drives has grown at an exponential rate. However, it has been found that these PWM drives cause premature failure of the magnet wire insulation systems used in such AC motors.
It is therefore highly desirable to provide an improved magnet wire for use in AC motors having variable frequency or PWM and/or inverter drives.
It is also highly desirable to provide an improved magnet wire which has increased resistance to insulation degradation caused by pulsed voltage surges.
The basic stresses acting upon the stator and rotor windings can be broken down into thermal stresses, mechanical stresses, dielectrical stresses and environmental stresses. All of these stresses are impacted by voltage, voltage wave forms and frequencies, in that the longevity of the winding is predicated upon the integrity of the whole insulation system. During the early stages of applying various voltages, voltage wave forms and frequencies to AC motors, the major focus was on the thermal stress generated by the unwanted drive harmonics passing through to the motor and the associated heating. The other critical factor dealt with the increased heating caused by reduced cooling capacity at slower speeds. While more attention was given initially to rotor bar shapes than to stator insulation voltage withstand capability, the present drive technology, which uses much higher switching rates (sometimes referred to as carrier frequencies) requires the focus to involve both the stator winding system and the rotor winding system.
The standard magnet wire used by most motor manufacturers is typically class H magnet wire. In accordance with the ANSI/NEMA magnet wire standard (ANSI/NEMA MW 1000-1993), this wire, under ideal conditions (twisted wire pair tests) is capable of a withstand voltage of 5,700 volts at a rise time not to exceed 500 volts per second. However, it has been found that utilizing current drive technology, a magnet wire may have to withstand voltage surges approaching 3,000 volts, voltage rises from about 1.0 kv per microsecond to about 100 kV per microsecond, frequencies from about 1 kHz to about 20 kHz, and temperatures for short periods of time approaching 300.degree. C. It has also been found that in certain circumstances, a surge is reflected so as to reinforce a primary surge wave voltage at succeeding coils to produce front times exceeding 3 micro seconds in subsequent coils.
These values are based upon the assumption that the wire film is applied concentrically to the conductor and that no appreciation of film thickness occurs in the manufacturing process or operation of the motor at high operating temperatures or that turn to turn bond strength may decrease significantly. Hence, coil movement and abrasion that reduce the thickness of the turn insulation over time can cause premature failure of the turn insulation.
Therefore, it is highly desirable to provide an improved magnet wire which can withstand voltage surges approaching 3,000 volts having rise times of less than 100 kV per micro second and temperature rises to 300.degree. C., frequencies of less than 20 kHz after the insertion of the windings in a motor rotor and stator at normal operating temperatures over the anticipated lifetime of the motor.
It is also highly desirable to provide an improved magnet wire which will pass the ANSI/NEMA magnet wire standards MW1000 and in addition ANSI/NEMA MG1-Parts 30 and 31 being developed for constant speed motors on a sinusoidal bus and general purpose induction motors used with variable frequency controls, or both, and definite purpose inverter fed motors, respectively.
A number of investigations to determine more accurately the voltage endurance levels of the present proposed insulation systems preliminarily indicate that the transient voltage levels combined with the operating temperatures of such motors can exceed corona starting levels. Some have blamed corona for the insulation failures in motors having variable frequency, PWM and/or inverter drives. Others have discounted corona as the culprit inasmuch as failure occurred in portions of the winding where the electrical field is low. While it is known that conventional enamels degrade when exposed to high voltage corona discharge, and that corona is discharged between adjacent windings of motor insulation, due to the inevitable voids and the high voltage ionization of air in the voids of the motor stator and rotor insulation windings, it has been found that insulation failure of motors driven by variable frequency, PWM and/or inverter drives is not primarily a corona insulation degradation mechanism.
Corona aging and magnet wire failure conditions may be distinguished from pulsed voltage surge aging and magnet wire failure conditions. Corona aging conditions occur in the presence of a gas (usually air in magnet wire windings) at positions of localized high electrical stress (AC or DC), that is strong enough to break down or ionize the gas, to produce electron or ion energy strong enough to break down polymer chains or to create ionic radicals via chemical reactions. The chemical reactions result in polymer degradation. Corona discharge is a relatively "cold discharge" and temperature is usually not a substantial factor. Magnet wire aging/failure due to corona is usually a long-term process.
In contrast, pulsed voltage surge aging and magnet wire failure does not require the presence of a gas media. Pulsed voltage surge failure instead requires repetitive or pulsed voltage surges having relatively short rise times, or high voltage to rise time ratios, relatively high frequency of pulse and relatively high impulse energy, and occurs in relatively high temperatures generated thereby. Given high voltages and minimum rise times, pulsed voltage surge failure can occur relatively quickly, and is believed to be the predominant cause of failure in variable frequency, PWM, and/or inverter driven motors.
It is therefore highly desirable to provide an improved magnet wire which meets all of the performance characteristics desired by motor manufacturers for stator and rotor windings for use under corona discharge conditions.
Finally, it is also highly desirable to provide an improved magnet wire which possesses all of the above-identified features.